The invention concerns a measuring device, especially a vectorial network analyzer. Network analyzers serve for the characterization of a device under test by the measurement of wave values, especially of S-parameters.
If the device under test is, for instance, a 2-port device, then, in a forward measurement, the wave which is projected toward a first port of the device under test, the reflected wave from the first port of the said device under test, as well as the wave which is transmitted through the device under test and exits at the second port, i.e., a forward transmitted wave, are measured. In a reverse measurement, the wave moving toward the second port of the device under test, the wave reflected from the second port as well as the wave transmitted through the device under test and emerging from the first port, i.e. backward-transmitted, are measured. From these measured wave values, various parameters may be computed, for instance, the so-called S-parameters, the reflection coefficient, the amplification or the input/output impedance (respectively). If only the amplitude of the waves are captured, then a scalar network analyzer is identified. If both the amplitude and the phase are determined, then this function is carried out by a vectorial network analyzer.
Such network analyzers are found in a wide variety of types in the state of the technology. A scalar network analyzer has been, for instance, disclosed by DE 198 57 834 A1. The network analyzer made known from this document serves for the measurement of a 2-port object and comprises two excitation/receiving units. The excitation/receiving units are each provided with a signal generator for emitting an excitation signal, so that the port of the device under test, which is connected to the said excitation/receiving unit, is stimulated by an excitation signal. By means of a directional coupler, first the excitation signal is diverted and respectively directed to a first receiving unit. Also, the signal reflected from the corresponding port of the device under test and transmitted through the device under test is likewise diverted and sent to a separate receiving unit.
The excitation frequencies of the signal generators are made adjustable separately for the excitation/receiving units by means of a processor using control signals. Nothing in the said document meets the question as to whether receiving apparatuses operate in accord with the superhetrodyne principle and whether the received signal is converted first into an intermediate frequency state. The necessary mixers are not described and the local oscillators required for this operation are not presented. For a scalar network analyzer, these are indeed not necessary. Most importantly, the document does not state, that these local oscillators necessary for the mixing in the intermediate frequency state are provided separately for the two excitation/receiving units and can be regulated separately from one another. The signal generators are in no way phase-locked or phase lock-controlled, and hence are suitable only for scalar network analysis and not for a vectorial network analysis.
Up to this time, it has not been possible to control the local oscillators of the different ports of the network analyzer in a separated manner, which oscillators are dedicated to the mixer of the receiving apparatuses, in such a way, that the oscillators of the individual ports can operate at different frequencies. This state of art has several disadvantages. Especially serious detriments are found in the so-called, “image-frequency” and in the so-called, “sum-frequency” problem. This subject will later be more closely discussed in detail with the aid of FIGS. 2, 3. The receiving of the image-frequency and sum-frequency, up to this time, could only be suppressed by a complex isolation receiver, which limits the measurement dynamics in the receiving paths, whereby the suppression is not sufficient for all purposes of a measurement. In this respect of insufficiency, where a frequency converting, device under test is involved, the simultaneous measurement of all forward-parameters and all backward-parameters is not possible, since the receiving apparatuses of the various ports can only simultaneously receive at a single frequency. Multi-tone measurements, especially, intermodulational measurements, can only be carried out with additional signal generators.
Consequently, the invention has the purpose of creating a measurement device, especially a vectorial network analyzer, wherein, in particular, the sum-frequence problem and the image-frequence problem are resolved.